Celluloses are the most abundant renewable resource on the earth and are environmentally friendly materials, so sufficient utilization of celluloses can not only protect environment but also save the limited unrenewable petroleum resources. However, celluloses are currently far from being sufficiently utilized in chemical industry, mainly because the current processes for dissolving cellulose are complex, costly and pollutive.
In the past one hundred years, conventional viscose process has been used for producing regenerated cellulose products such as rayon, glassine paper and the like. The conventional viscose process comprises reacting cellulose with CS2 (33 wt %) in the presence of strong base (the concentration of sodium hydroxide being 18 wt %) to produce cellulose xanthate that is dissolved in the alkaline solution to form a viscose solution, and then spinning or casting the viscose solution of cellulose, followed by regenerating in diluent acid solution to obtain viscose fiber (rayon) or glassine paper. A great quantity of toxic gases such as CS2 and H2S which severely pollute environment are released during the process and are harmful to human health (J. Macromol. Sci.-Rev. Macromol. Chem., 1980, C18 (1), 1).
In the prior art, the cuprammonium process for producing cuprammonium rayon also has drawbacks of environmental pollution, high cost and difficulty to recover solution. The processes in which other organic or inorganic solvents such as dimethylsulfoxide-nitrogen oxide (U.S. Pat. No. 3,236,669, 1966), aqueous ZnCl2 solution (U.S. Pat. No. 5,290,349, 1994), LiCl/DMAc (U.S. Pat. No. 4,302,252, 1981) and the like are used, respectively, are difficult in industrialization due to the cost and their complicated dissolving procedures.
N-methylmorpholine oxide (NMMO) (U.S. Pat. No. 2,179,181, 1939; U.K. Patent No. GB1144048, 1967; U.S. Pat. No. 4,246,221, 1981) is considered as the most promising solvent for cellulose so far. In 1989, Bureau International pour la Standardisation des Fibres Artificielles (BISFA) in Brussels named cellulose fibers made by such NMMO process as “Lyocell”. Although a small amount of products of cellulose fibers made thereby had been marketed, the industrial production of them developed slowly due to high cost and high spinning temperature.
In addition, a process had been proposed that comprises reacting cellulose with urea at high temperature to obtain cellulose carbamate, and then dissolving directly in a diluent alkaline solution to obtain spinning solution (Finland Patent No. FI61003; Finland Patent No. FI62318; U.S. Pat. No. 4,404,369). However, this process requires a great amount of urea, leads to side product(s), and is difficult for industrialization either. Japan Patent No. JP1777283 disclosed that cellulose was dissolved in 2.5 mol/L aqueous NaOH solution, but only wood pulp cellulose having a polymerization degree of below 250 and being treated by vapor explosion could be used, which could be dissolved in such aqueous NaOH solution at about 4° C. The cellulose filaments made by using this process have a poor strength and are not suitable for spinning or film-forming in industry.
The present applicant proposed in Chinese Patent No. 00128162.3 that a mixed aqueous solution of 4 wt %-8 wt % sodium hydroxide and 2 wt %-8 wt % thiourea was used to, after being cooled, directly dissolve at room temperature the natural cellulose having a viscosity average molecular weight of less than 10.1×104 and the regenerated cellulose having a viscosity average molecular weight of less than 12×104 to obtain transparent cellulose solution. However, the practices indicated that the solvent system must be kept under freezing condition (−20° C.) for 3-8 hours to form an ice-like stuff and then thawed before it was used to dissolve cellulose for preparing transparent concentrated cellulose solution. Thus, it is applicable to laboratory scale only at present, and is not suitable for industrialization.
In addition, the present applicant proposed in Chinese Patent No. 200310111447.8 that a mixed aqueous solution of 8.1 wt %-12.0 wt % sodium hydroxide and 4.0 wt %-6.0 wt % thiourea was used for directly dissolving cellulose, and a process of using this mixed aqueous NaOH/thiourea solution for preparing regenerated cellulose films or fibers in laboratory scale, but this process was merely provided for research and was not suitable for industrial production.